To evaluate methionine requirement when cystine is provided by diet. Methionine balance (at requirement intake) will be compared with lower methionine intake plus cystine. By administering orally multi-labeled isotopes of methionine and cysteine, the investigators will be able to obtain information on methionine-cystine cycle and further investigate the cystine sparing role in methionine utilization. During an additional tracer study, tracers will be infused, I.V., to provide information related to the obligatory oxidative losses of methionine and to the efficiency of the utilization of methionine by the human body.